The inner surface of a tube made of aluminum that constitutes refrigerant passages of a heat exchanger for automobiles is required to be highly corrosion-resistant, because the inner surface is always in contact with a refrigerant. For this reason, the inner surface of the tube is lined, for example, with a corrosion-resistant material or a sacrificial material.
Specifically, an example of the lined tube is one in which the inside of a core for tubes made of high-strength JIS-3003 alloy (Al/0.15 wt. % Cu/1.1 wt. % Mn) or the like is lined with JIS-7072 alloy (Al/1 wt. % Zn) material, which latter is excellent in corrosion resistance.
The thickness of these conventional tubes of aluminum used for heat exchangers for automobiles is on the order of 1 to 2 mm, and these tubes are produced by preparing a clad pipe using a composite hollow billet by extruding and drawing the clad pipe into a tube.
In recent heat exchangers for automobiles, the wall of the tubes tends to be made thin, as the heat exchangers are made light in weight and the cost of the heat exchangers is lowered. As the method for producing these tubes that are required to have a thinner wall, as well as being required to be corrosion resistant and plastically workable, the use of clad tubes by the extrusion production method is useful.
In the extrusion production method, however, air is apt to be retained between the skin material and the core, and between the core and the lining material, and hence blisters (defects caused by air which was involved into the interface of the clad billet during its preparation and was expanded during extrusion and subsequent working and heating processes to cause the clad billet surface to blister.), defective joining, debonding of the inner pipe, and the like are apt to occur. These defects hardly cause problems in regard to the function in the case of conventional thick-wall tubes, whereas all of these defects are problems in the case of thin-wall tubes. For example, defective joining of an outer pipe may cause cracking in the drawing step and cracking in the duration of the post-working of the pipe of the heat exchanger parts, and debonding of an inner pipe leads to debonding of the lining material as the sacrificial material, and as a result the core is exposed there and will become corroded, to form through-holes. Furthermore, there is a problem that such debonding of an inner pipe for an elongate extruded material and an elongate coil produced by drawing a coil is hard to inspect before delivery.
As one means for preventing those problems, for example, a method is known in which a two-layer billet having a core and a lining material is cast previously in a casting stage, but the cost of the production is high and the method is technically difficult. Further, as a method wherein billets combined at low temperatures are used, there is a method in which the outer diameter of an inner billet is increased by moving forward a mandrel during the extrusion, to bring it in close contact with a core (JP-A-3-23012 ("JP-A" means unexamined published Japanese patent application)), but this method does not result in a satisfactory effect for thin-wall tubes.